With many off-the-shelf machines, the OEM copies of Windows are tied to the mobo/hardware ID.
These copies of Windows don't need an install key.  Once loaded, they authorize based on mobo/hardware ID.
 
OEM copies of Windows that you buy from retailers have an install key (not tied directly to hardware).

Jim Roseberry
This last migration, the Microsoft agent wanted a receipt for the copy of Windows.  
I'm like... uhhh... I've got the copy right in front of me (jewel-case and all).
Had to take a picture of the Install Code... and leave that on the desktop for the agent to authenticate. 
Minor annoyance... but all was fine.  
 

 
Was this Windows 10?  I got the free upgrades and all of my licenses are retail. I'm thinking of donating an Intel system without the drives and piecing back together a Phenom system and using that W10 OS.

You may need to contact a Microsoft rep to authorize Windows 10... but that's the worst case scenario.
You'll definitely be able to use the license.

my understanding of the original terms when w10 was being offered,....errrr, pushed actually, for "free" the price was surrendering the lisc and terms of of the OS being replaced to that of W10- the implication being: you cant go back. If that is the case idk, but I would want to ensure I wouldnt be putting myself into a corner if things arent working as they should. My investment into daw and vst is for my enjoyment, not aggravation.

LaszloZoltan
my understanding of the original terms when w10 was being offered,....errrr, pushed actually, for "free" the price was surrendering the lisc and terms of of the OS being replaced to that of W10- the implication being: you cant go back. If that is the case idk, but I would want to ensure I wouldnt be putting myself into a corner if things arent working as they should. My investment into daw and vst is for my enjoyment, not aggravation.

That's correct, that when you upgraded to Win 10 for free, you surrendered your previous OS license.  Not true if you purchased a new license.  MS gave you a period to roll back from the free upgrade, initially 30 days, but it has been shortened since the free period started.
 
If you are uncertain, it would be better to buy a fresh copy of Win 10, so you could still use your old license if you wish.
 
Regardless, that issue has nothing to do with a motherboard swap.  You can still do those.

This has worked for me in the past but a clean install is always best.
 
https://arstechnica.com/c...ic.php?f=8&t=42051